Transthoracic lung aspiration was performed in 30 episodes of pneumonia in 27 children with malignancy on chemotherapy to assess etiology of pulmonary infections. Total of 22 organisms were isolated in 16/30 (53.3%) episodes. No acid fast bacilli or Pneumocystis carinii were seen. Organisms grown by blood culture correlated with that of lung puncture in 5 episodes, while throat culture and nasopharyngeal organisms correlated with that of lung puncture on one occasion each. Organisms isolated in 8/18 episodes (44.4%) of antemorten transthoracic aspiration included: Pseudomonas aeruginosa, Klebsiella pneumoniae, Escherichia coli, Streptococcus faecalis and Diphtheroids. In 3/18 episodes, lung puncture results altered treatment and thus resulted in survival of the patients. Only one minor complication occurred in this study--pneumothorax that resolved spontaneously. Thus, transthoracic lung puncture is an useful and safe procedure in immunocompromised patients with pneumonia who do not respond to initial broad spectrum antibiotics.